Manhattan Correctional Center officials said Roberto Grant (r.) died of a drug overdose — but his family says he was beaten to death in a cell. (Pearl Gabel/Pearl Gabel/ New York Daily News)

Manhattan federal jail officials tried to "cover up" the fatal beating of an inmate, by falsely blaming his death on drugs, a new $20 million lawsuit alleges.

Metropolitan Correctional Center staffers allegedly told Roberto Grant's family that the 35-year-old died of a drug overdose at the facility on Park Row in downtown Manhattan on May 19, 2015.

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The city medical examiner's office later found there were no drugs in his system, and he suffered "blunt force trauma," according to his family's suit filed in Manhattan Federal Court Wednesday.

A copy of Grant's autopsy, obtained by the Daily News, lists the cause and manner and death as "undetermined," but does indicate he suffered from a blunt force trauma.

The suit contends Grant was "beaten to death within a ...Multi-Inmate Jail Cell" and that authorities "failed to afford reasonable oversight, supervision and security" which contributed to Grant's death.

After that, court papers charge that people involved "did attempt to cover-up" what happened.

"I was like, 'How could they lie to me? How could they do this to me?'" his mother, Crecita Williams, told the Daily News. "My son was my heart."

Williams said she is raising his twin girls, who are 4-years-old, and grappling with the questions they ask about their dad.

"I don't know how to answer," said Williams, 57. "I try to answer as best I could."

Grant pleaded guilty in August 2014 to his involvement in five jewelry store robberies.

The suit, filed by Andrew Laufer, seeks at least $20 million in damages.